I did the first job, but still have a second site to look at. The last job I set to 30 seconds.
Relays, mains fail, delay.
- Thread starter Lectrician
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What did you use in the end?
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Use Hermes, they're a lot cheaper.
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How very steampunk.
Wouldn't work reliably in Summer hot months though.
The time delay would vary with the ambient temperature, yes. But as long as the switching point was above the maximum possible ambient temperature it would still work. The heater temperature would increase with ambient temperature so there will always be a differential. A more complex thermostat could sense the differential temperature rather than an absolute and that would be more consistent.
How about a heater element in a pressure vessel. As long as the heater is on, water vapour is generated and this operates a pressure switch or condenses on a bathroom humidistat. If the power fails, the steam supply ceases after the water cools and the switch is released.
I am an electronics engineer and couldn't have come up with a better idea myself, I think you need to patent your concept before the Chinese steal your idea and flood the market with cheap and cheerful timers.
Unfortunately they already exist (the bimetallic version).
You can get electronic versions these days but I expect they mostly need a continuous power source.
OP: try searching for time delay relays. RS seems to have over 1000 types!
You can get electronic versions these days but I expect they mostly need a continuous power source.
OP: try searching for time delay relays. RS seems to have over 1000 types!
Indeed - but, as you imply, I don't think one would be able to get any significant 'off delay' (on power failure' without some sort of back-up power source).
Alien though it might seem to an electrical or electronics engineer, a thermo-electric (like the bimetallic strip thermostat) or electro-mechanical (relay with a clockwork off-delay) would seem an ideal solution that does not rely on a backup power source always being present and working. As has been implied, there would be other (e.g. pneumatic or hydraulic) non-electrical approaches.
Kind Regards, John
Oh well, you say it, but perhaps you didn't dwell deeper into it, you can rig a small circuit with rectification and using a storage capacitor to store a charge that slowly drains away holding a low voltage relay for as long as 30 seconds, and once the power is restored another timing circuit powers up and starts timing up and flicks another relay to summon text message power reso#tored thus not requiring any back up power. And this would be based on electronic as well as simple RC timing. (Possibly using a low voltage transformer to run the electronics for restore timing.)
I did consider it, but without doing any sums (perhaps I should have done!) felt that it would probably need a very large capacitor to keep a relay energised for 30 seconds.
Having now done some sums, it looks as if it would certainly be do-able with a reed relay. If one used a 5V reed relay with a coil resistance of 500Ω and a drop-out voltage of 0.6V, then one ought to be able to get about 29 seconds with a 27,000 μF capacitor - quite a large animal, but nevertheless readily available. With anything other than a reed relay, one would obviously need a larger capacitor - for example, it looks as if a 5V relay with a 100Ω coil (and 0.6V drop-out) would need something like a 150,000 μF capacitor.
However, one would still have to include a rudimentary 5V (or whatever) PSU, something not needed for the non-electrical options
Kind Regards, John
Using a 24 volt reed relay woth its lower operating current would require a smaller capacitor. Some large electrolytes display a battery like function and self "recharge" to a few volts after being discharged. It is not impossible that when in "battery" mode a large capacitor could hold a relay in for far longer that calculations would suggest.
Yes, if you used a relay with a greater coil resistance than that of a 5V reed relay I used as my example (about 500Ω), then a smaller capacitor would be required for the same time delay (assuming that the ratio of normal operating voltage to drop-out voltage was similar).
However, even with a 24V reed relay, one would have to look around to find one which would give as much advantage as one might think. Although the coils of 24V reed relays are typically 1.5-2kΩ, some are as low as 600Ω (which would offer little advantage over a 5V one). With a 2kΩ coil and a 10% dropout voltage, something like a 6,800 μF theoretically ought to give around 30 seconds.
I haven't looked, but I doubt that traditional (not reed) 24V relays would offer much, if any, advantage over a 5V reed one.
Mind you, one obviously doesn't have to stop at 24V!
Yes, that's possible, but I think it would be pretty unpredictable - probably OK for the purpose under discussion, but probably no good if one wanted a fairly well-defined delay period.
Kind Regards, John
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How about an electromechnical thermal overcurrent HV protection relay?
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